I think that flags are marked as "helpful" when one user has agreed with them, so the question may have one close vote from the review item you raised but not yet enough to close it. The close review queue is at 11k right now and I've not often seen it lower than that -- stuff takes time.
– TZHXFeb 24 '15 at 8:53

1 Answer
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Note that flagging a post as a duplicate only puts the question into the closing review queue. It is users with the close vote privilege that then actually close the question.

Those users could have voted to leave a question open or could have voted to close the question for different reasons.

Now that the post has been deleted, however, no one can vote close it. There is no need to. Your flag has been marked as helpful automatically when the post was deleted; the system assumes your flag had merit if the post was removed altogether rather than closed.

@ImranAliKhan: One vote by a user with the close vote privilege that agrees with your flag can also mark it as helpful. There are currently 2 dupe close votes on that post; at least one of which must've been cast after your flag (there could have been more that have expired since).
– Martijn Pieters♦Feb 24 '15 at 9:32